1. Field of the Invention
The present invention relates to improved TFTs (thin film transistors) and a manufacturing method thereof. More specifically, the present invention relates to TFTs which obtain a high drain current at a low voltage, and a manufacturing method thereof.
2. Description of the Related Art
FIG. 1 shows a configuration of a conventional n-channel MOSFET (metal oxide semiconductor field-effect transistor).
A TFT is a kind of MOSFET, wherein semiconductor thin films of amorphous silicon or polycrystalline silicon are formed on a glass substrate, and the FET structure is generated on the semiconductor thin films.
As shown in FIG. 1, electrical conduction on a current path (i.e., a channel) between a source, which corresponds to a cathode, and a drain, which corresponds to an anode, is controlled by a voltage VGS of a gate contacted at an oxide film on the channel in the MOSFET.
The source and the drain are formed by generating two n-type layers 20a and 20b on the p-type silicon substrate 10. The gate includes three layers of metal, oxide, and semiconductor, and an electrode 30 is formed on an oxide film 40 between the source and the drain on the substrate.
Basically, On/Off states between the drain and the source of the TFT are controlled by a voltage supplied to the gate, which is a third electrode. No current flows between the drain and the source in the Off operation, and the current flows therebetween in the On operation. When the gate voltage becomes greater than a predetermined voltage (a threshold voltage), the transistor is turned on.
The operation region of the TFT is divided into a linear region and a saturation region in the same manner as the MOSFET. When the drain voltage is small, the drain current is proportional to the drain voltage since the characteristics between the drain and the source give ohmic characteristics. When the drain voltage becomes large and the gate voltage is neutralized, the channel is pinched off from the drain, and the drain current no longer increases and has a constant value irrespective of increase of the drain voltage.
When the potential of the source n-type layer 20b is set as 0, and when the drain current is saturated, the drain voltage VDS corresponds to a difference between the applied gate voltage VGS and the threshold voltage Vth. Therefore, a high drain voltage VDS is required so as to obtain a stable saturation current in the drain region in the case of the polycrystalline silicon TFT on the glass substrate. However, power consumption also increases as the drain voltage VDS increases. Low power consumption of portable devices such as mobile stations or PDAs is an important technical target in the corresponding field, and it is essential to save the power for driving organic displays in the portable device.